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Old 5th August 2013, 09:51 PM   #21
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Quote:
Originally Posted by Nigel Goodwin View Post
If you unplug V1 does the hiss disappear? - it's usual to only use a low noise pentode for the first stage.
Unlugging V1 makes it virtually all disappear. I have to put my ear up to the speaker to hear it. Adding one valve should not be too much of a problem. there will be 1/2 over, however. Perhaps something can be done with it... In fact, I already know: another gain stage for Normal Clean; it's too low going into the FX loop. A booster in front or a more complicated level control would have to be added. This was done on a Soldano SLO-100 for Eric Clapton.

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Originally Posted by Nigel Goodwin View Post
Feedback reduces gain, and increases quality - presumably two things you don't want to do?.
True. The gain with local feedback is 10 at best. No good for V1 and V2.

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Originally Posted by Nigel Goodwin View Post
However, removing the capacitors on the cathodes will give you local negative feedback, the capacitors also seem rather small? (although I haven't bothered doing the calculations), which would give higher gain at higher frequencies and make the hiss worse.

So try disconnecting the cathode capacitors one at a time and see what effect that has.
Decreasing those does reduce gain. Increasing them increases gain at lower frequency points, but still allows all the highs in, if I understand correctly:

PartialBypass.jpg

For the EF86 Pentode idea: The Valve Wizard -Small Signal Pentode

Thank you for all these ideas.
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Old 5th August 2013, 09:57 PM   #22
tinitus is offline tinitus  Europe
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Originally Posted by Nigel Goodwin View Post
You don't play heavy metal do you?
ehhh .... did you say hiss
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Old 5th August 2013, 10:03 PM   #23
tinitus is offline tinitus  Europe
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Originally Posted by ITPhoenix View Post
Decreasing those does reduce gain
I suppose it will only reduce the lower frequencies that does not produce hiss

so, basicly it only means increasing the hiss, in relative terms


btw, pardon if its a silly question, but is not usually the input tube that has no cathode bypass cap, and the output tube that has a bypass cathode cap ?
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Old 5th August 2013, 10:19 PM   #24
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Originally Posted by tinitus View Post
is that a normal situation
Not for me. I have to use one 20w 5 inch speaker at home. I do need the gain, though. I would like to achieve an almost square wave. It has limited use, but can be "softened up" with reverb or echo.

Example of near square wave (probably electronic overdrive): Bruce Springsteen - Badlands (Live In Phoenix '78) [The Promise] - 1080p HD! - YouTube The lead starts at 2:33.
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Old 5th August 2013, 10:27 PM   #25
tinitus is offline tinitus  Europe
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I do need the gain, though.
so you are not using the upper tube, V1A ?

and further, you also have two channel output ?

I don't know much
but I got less noise when I left the 2 channel output, and settled with just one channel
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Old 5th August 2013, 10:32 PM   #26
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so you are not using the upper tube, V1A ?
V1A is the Normal channel; Clean or Crunch. It is out of the chain during OD, as LDR5 would be open.
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Old 5th August 2013, 10:41 PM   #27
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Originally Posted by tinitus View Post
so you are not using the upper tube, V1A ?

and further, you also have two channel output ?

I don't know much
but I got less noise when I left the 2 channel output, and settled with just one channel
The Normal channel in Crunch is much quieter than OD. I would not consider making only one channel. In fact, I would like to have two FX loops and two tone stacks; a completely separate chain for each channel!

This way, you could put two different boxes (or more) in each loop, dial in all the settings for optimum and just toggle the footswitch. If you want the same effect on both channels, two separate boxes would be needed.

This scheme is more complicated and more expensive, but it also solves many problems.
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Old 6th August 2013, 07:41 AM   #28
ivan H is offline ivan H  Australia
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Hi, now I' no expert on any of this but I've built a few tube guitar amps, most recently a 50 watt, footswitchable Caswell #39/ Levi #36 type thing (as used by Slash on appetite for destruction) which I suppose is kinda hi gain. Now I think ur hiss is coming from the cathode bypass value used for the 1st stage. The value of the cathode bypasd cap is used to tune that stage's frequency responce, & ur chosen cathode values of 1k8/0.1uf gives a roll off point of around 884hz. U said about Marshall using a 0.68uf, but coupled with a 2k7 resistor this gives a roll off point of 86.69hz. If ur after a similar 3db roll off point using a 1k8 resistor, use a 1uf for a roll off at a slightly higher 88.4hz. U wont get the same "raw" kind of sound as a Marshall & it'll probably also feel a bit tighter. The Marshall' raw sound & looser feel come from using the higher 2k7 resistor value. Cheers

Last edited by ivan H; 6th August 2013 at 08:11 AM.
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Old 6th August 2013, 07:50 AM   #29
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Originally Posted by ivan H View Post
Now I think ur hiss is coming from the cathode bypass value used for the 1st stage. The value of the cathode bypasd cap is used to tune that stage's frequency responce, & ur chosen cathode values of 1k8/0.1uf gives a roll off point of around 884hz.
It is 1uf. And whether it's increased to 11uF or removed completely, the hiss is still there. The hiss sounds like white noise. I will try a 5751 in that position and see what happens. Also, I will try some supposedy quieter 12ax7s for V1 and V2.

The 6N2P-EV is heavy duty for military use. None of the ones I have are microphonic, and all have the same performance. Who knows if they are less noisy than other types, unless they are replaced?

Even if it was 0.1uF, the high frequencies would still come through. I provided a chart above.

Thank you for your input.
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Last edited by ITPhoenix; 6th August 2013 at 07:54 AM. Reason: Forgot something....
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Old 6th August 2013, 08:32 AM   #30
ivan H is offline ivan H  Australia
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Ok, my appilogies, I thought it said .1uf, I'm veiwing it on a phone. Just while I,m here, the bright Marshall channel can sometimes be a little prone to hiss. Resistor types, lead dress & especially the tube can all contribute. When I first fires my last build up (the circuit is know to be prone to squeal), I found it had hiss & squeal at higher gain settings. Luckilly it was only the 1st tube, the troubles went when I replaced it. Hopr u gey it sorted. Cheers
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